This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, the invention relates to decoupling shields particularly useful in an NMR apparatus for decoupling radio-frequency (RF) and gradient coils.
In general, the NMR apparatus includes a main magnet typically configured as a solenoid having a bore formed therein for accepting the NMR sample to be studied. The magnet which is frequently of a resistive or superconductive design is used to produce a homogeneous polarizing magnetic field in a predetermined region within the bore. The magnetic field is needed to polarize the nuclei within the sample, so that the NMR phenomenon can be excited. Magnetic fields utilized in NMR imaging and particularly those needed for in-vivo spectroscopy must be highly homogeneous to, for example, in excess of one part in a million. To achieve such degree of homogeneity, auxiliary field-correcting coils referred to as shim coils are provided in the magnet bore to provide the necessary correction factors. Radio-frequency coils constructed on cylindrical forms are positioned within the magnet bore coaxially with the longitudinal bore axis. The RF coils are used to irradiate the sample studied to excite the nuclear spins. Upon cessation of the RF excitation, the excited nuclei radiate an NMR signal which is detected with the same coil used to excite the resonance or with a separate coil orthogonally oriented with respect to the excitation coil. Additionally, in an NMR imaging apparatus three sets of gradient coils are provided within the bore. The gradient coils when energized are capable of producing three orthogonal, substantially linear magnetic field gradients each oriented along one of the directions of a conventional Cartesian coordinate system. The gradients are utilized in a well-known manner to encode into NMR signal spatial information of the nuclear spin distribution within the sample region excited by the RF coils. This information can be used to reconstruct images displaying one or more NMR parameters.
Bore space in NMR magnets utilizied in whole-body NMR imaging is limited due to the fact that sufficient clearance must be provided to accommodate the human torso. The RF, gradient, and/or magnet shim coils are therefore situated in close proximity to one another. In a typical configuration, the RF coils are placed coaxially within the gradient coils. There can be numerous interactions between various coils which can degrade the performance of the RF coils. The gradient coils or shim can cause losses to occur in the RF coils. These losses can lower the quality factor Q of the RF coil resulting in a lower signal-to-noise ratio in the image. Although the signal-to-noise ratio can be improved by signal averaging, this is not a desirable solution since the NMR data collection time is unacceptably increased. The numerous windings within the gradient coils are coupled together by stray capacitances which can give rise to a plurality of spurious resonances when the RF coils are energized. If one or more of these spurious resonances interacts with the RF coil, the desired RF frequency may be displaced and the desired resonance damped. These detrimental effects increase with the proximity of the RF coils to the gradient coils.
The interaction between the RF and gradient coils can be reduced or eliminated by placing an RF opaque screen between the gradient and RF coil forms. The screen must be several RF skin-depths thick to decouple the coils effectively. Suitable screens have been fashioned from copper mesh screen or copper foil having a thickness of 0.004 inches with a fiber backing. Such screens have been found effective in destroying RF interaction between the RF and gradient coils. A drawback associated with such simple shields is that the high-frequency response of the gradient coils is degraded. The rise time of the switched gradient field is lengthened by the eddy current induced in the continuous conductive layer of the RF shield.
It is, therefore, an object of the invention to provide an RF shield for effectively decoupling the RF and gradient coils without significantly degrading coil performance.
It is another object of the invention to provide an RF shield which acts as a low-pass filter for the time-dependent magnetic fields produced by the gradient coils.
It is a further object of the invention to provide an RF shield which is substantially transparent to homogeneous magnetic field and audio frequency gradient magnetic fields but which is highly reflective with low loss for RF magnetic fields.